Is there any specific reason that n395an (AA OneWorld 763) and n171dz (DL habitat for humanity 763) have not been to JFK since the beginning of May, and not very often before that? It seems that there are certain planes, not just special colorahcemes, but some regular aircraft do not visit certain airports. Is there a reason?

Depends on the airline and fleet, but most a/c free-flow within the network. The primary factor in a/c routing is typically maintenance. In some cases, especially with long-haul a/c, planes may bounce around a specific theater for a while before being routed elsewhere. For example, N171DZ spent most of the past 1-2 weeks flying within the Pacific region. But, as it happens, it'll be in JFK tonight. (Coming in from ATL, out to IST)

I don't spend a lot of time checking up on them, but that's probably not unusual for DL 763s in the Pacific because there aren't that many places for them to bridge to the Atlantic/Latin America networks.